Viscous fluid couplings have received wide acceptance in the automobile industry for controlling the amount of torque transmitted to a radiator cooling fan. The most common form of such viscous fluid couplings is the air temperature response type such as illustrated in U.S. Pat. No. 3,055,473. In certain applications, however, it has become desirable to directly sense the temperature of the liquid coolant entering the radiator rather than the temperature of the air passing through the radiator. To date, many arrangements have been proposed to accomplish this result. Typically, these arrangements have made use of wet or dry plate clutches that are pneumatically actuated or electromagnetic clutches that are electrically actuated. A major shortcoming of many prior art magnetically actuated clutches or couplings resides in the fact that relatively high torque levels are involved. These high torque levels require relatively massive engagement mechanisms which must be engaged through the action of intense magnetic fields set up by large and expensive inductors. Such inductors draw substantial amounts of energy from the associated vehicle's electrical system and thus reduce overall operating efficiency. Additionally, such devices typically employ relatively large springs which bias a portion of the engagement mechanism toward either the engaging or nonengaging position. Accordingly, the magnetic field must also overcome the force of the biasing spring as well as the mass of the engagement mechanism.
More recently, improved prior art electromechanically actuated viscous fluid couplings have been suggested which employ inductors to establish magnetic fields which activate valves controlling the flow of viscous fluid within the clutch. This arrangement represents an improvement inasmuch as the magnetic field established by the inductor only had to be large enough to move a relatively small valving arm and biasing spring. Although representing an improvement, such devices had inherent inefficiencies inasmuch as their valving arm biasing springs had to be large enough to overcome the kinetic energy of the viscous fluid flowing thereby.
Most recently, advanced prior art electromechanically actuated viscous fluid couplings have proposed providing valving elements which are mounted for rotation with one of the viscous clutch members which operate to frictionally engage the other member to extract kinetic energy therefrom to move the valving element and thereby vary the torque transmitted between the clutch members. This arrangement is desirable in a simple "ON" and "OFF" type clutch. However, because the amount of kinetic energy to be extracted is related to the differential speed of the clutch plates, it is apparent that a speed differential is not present during all operating modes of the fan and thus an unacceptable time lag may occur between the time a control signal input is received and the clutch responds. This problem is particularly acute in the case of continuously variable fan drives where an adequate energy source for movement of the valving member must be available at all times during operation.
Although adjustable speed fan drives are known, they tend to cycle and produce objectionable noise as well as waiver or hunt in speed about a desired operating point. To ensure adequate cooling, they generally are operated at an excess speed to the optimum thereby consuming excessive energy and reducing overall efficiency.
It will be apparent from a reading of the specification that the present invention may be advantageously utilized with fluid couplings intended for many different applications. However, the invention is especially usefull when applied to a viscous fluid coupling which serves as a drive for the radiator cooling fan of a vehicle engine, and will be described in connection therewith.